1. Field of the Invention
The present invention relates to a scroll type compressor and more particularly, to an improvement for making a compressor more compact.
2. Description of the Related Art
Conventional scroll type compressors (hereinafter simply referred to as "compressors"), have a fixed scroll that is fixed in a shell and an orbiting scroll that is supported for revolving movement in the shell. The fixed scroll includes a fixed end plate and a fixed spiral element formed integrally with one side of the fixed end plate. The inner and outer walls of the fixed spiral element form involute curves. The orbiting scroll includes an orbiting end plate and an orbiting spiral element formed integrally with one side of the orbiting end plate. The inner and outer walls of the orbiting spiral element also take the form of involute curves. The fixed spiral element and orbiting spiral element are joined with the phase of the latter spiral element shifted by 180.degree. from that of the former spiral element. A compression chamber is therefore formed between the scrolls.
In a compressor of this type, rotation of a drive shaft causes revolution of the orbiting scroll. Consequently, the compression chamber moves toward the center while its volume is decreased, thereby discharging a compressed fluid into a discharge chamber.
Further, as shown in FIG. 4, the inner wall of a fixed spiral element 82 from a tip portion 82b to a base portion 82a is formed along an inner involute curve I.sub.in. The outer wall of the fixed spiral element 82 is formed along an outer involute curve I.sub.out. This outer wall extends from the tip portion 82b to a position where the involute angle of this position is smaller by almost 180.degree. than that of the base portion 82a. Since the outer wall of the fixed spiral element 82 is connected to an arc E that forms the inner wall of a shell 81, the fixed spiral element 82 is connected integrally with the shell 81. The inner and outer walls of an orbiting spiral element 83 are likewise formed along the involute curves I.sub.in and I.sub.out, respectively.
According to this compressor, the orbiting spiral element 83 and fixed spiral element 82 must be made to contact each other within a predetermined involute angle in accordance with revolution of the orbiting scroll in order to form the compression chamber. The center O of the shell 81 is designed to be coincident with the center S.sub.O of an involute generating circle S for the fixed spiral element 82. In addition, the center P.sub.O of an involute generating circle P for the orbiting spiral element 83 moves on a revolution circle C concentric to the center O (center S.sub.O) of the shell 81, permitting the orbiting scroll to revolve.
However, when the center O of the shell 81 coincides with the center S.sub.O of the involute generating circle S for the fixed spiral element 82, a wasted space will be formed between the inner wall of the base portion 82a of the fixed spiral element 82 and the inner wall of the shell 81. This will be discussed more specifically below.
A distance W.sub.8 between the inner wall of the base portion 82a of the fixed spiral element 82 and the inner wall of the shell 81 is expressed by the following equations: EQU a+W.sub.8 =R.sub.or +a+t+c EQU W.sub.8 =R.sub.or +t+c
where
t: is the thickness of the base portion 83a of the orbiting spiral element 83, PA1 c: is the minimum clearance between the outer wall of the base portion 83a and the inner wall of the shell 81, PA1 a: is the distance between the center P.sub.O of the involute generating circle P for the orbiting spiral element 83 and the inner wall of the base portion 83a of the orbiting spiral element 83 (=distance between the center S.sub.O of the involute generating circle S for the fixed spiral element 82 and the inner wall of the base portion 82a of the fixed spiral element 82, and PA1 R.sub.or : is the radius of orbital revolution.
The minimum diameter D.sub.8 of the shell 81 is therefore expressed as follows: EQU D.sub.8 =2(a+R.sub.or +t+c).
This conventional type of compressor therefore has a wasted space formed inside, increasing the diameter of the shell 81, which inevitably requires larger space to mount the compressor in a vehicle or the like.
One attempt to reduce this shortcoming is the compressor shown in FIG. 5, which is disclosed in Japanese Unexamined Patent Publication No. 55-51987. In this compressor, the center O of a shell 91 is shifted by R.sub.or /2 from the center S.sub.O of the involute generating circle S for a fixed spiral element 92 in a direction opposite to the direction toward a base portion 92a of the fixed spiral element 92. In the compressor disclosed in this Japanese publication, the inner and outer walls of the fixed spiral element 92 and an orbiting spiral element 93 are also formed along the involute curves I.sub.in and I.sub.out, respectively. As the center P.sub.O of the involute generating circle P for the orbiting spiral element 93 moves on a revolution circle C concentric to the involute generating circle S for the fixed spiral element 92, the orbiting scroll revolves.
With t, c, a and R.sub.or defined as given above, a distance W.sub.9 between the inner wall of the base portion 92a of the fixed spiral element 92 and the inner wall of the shell 91 is expressed by the following equations: EQU R.sub.or /2+a+W.sub.9 =R.sub.or -R.sub.or /2+a+t+c EQU W.sub.9 =t+c
The minimum diameter D.sub.9 of the shell 91 is expressed by: EQU D.sub.9 =2(a+R.sub.or /2+t+c).
This compressor can therefore reduce the wasted space by an amount expressed by the following equation as compared with the above-described typical compressor. ##EQU1## Likewise, the minimum diameter of the shell can be reduced by an amount expressed by the following equation. ##EQU2## As apparent from the above, the compressor can be made more compact, so that this compressor is more easily mounted in a vehicle or the like than the aforementioned compressor.
However, the compressor disclosed in the above publication still has a wasted space W.sub.9 expressed by the formula: EQU W.sub.9 =t+c
The wasted space is between the inner wall of the base portion 92a of the fixed spiral element 92 and the inner wall of the shell 91. The minimum diameter of the shell 91 is thus limited to: EQU D.sub.9 =2(a+R.sub.or /2+t+c)
Accordingly, the disclosed compressor is not an adequate solution to the wasted space problem.